1. Field of the Invention
The present invention relates to an active matrix liquid crystal display device.
2. Description of the Related Art
Heretofore, active matrix liquid crystal display devices capable of displaying color images have been of a structure including a TFT (Thin-Film Transistor) substrate with TFTs and pixel electrodes disposed thereon in association with respective pixels, an opposing substrate with color filters and a common electrode disposed thereon, and a liquid crystal layer sealed between the TFT substrate and the opposing substrate. In this structure, the color filters and the pixel electrodes need to be positioned accurately in alignment with each other. In order to prevent an unwanted leakage of light, a light shielding layer referred to as a black matrix is required to be positioned between the color filters which are combined with the respective pixels on the opposing substrate. In view of these requirements, it has been proposed to fabricate color filters on a TFT substrate. With color filters fabricated on a TFT substrate, an opposing substrate can be constructed of a transparent substrate and a transparent common electrode fabricated uniformly over the transparent substrate. Therefore, the process of manufacturing active matrix liquid crystal display devices is simplified, and it is relatively easy to achieve precise alignment between the opposing substrate and the TFT substrate. In addition, various interconnections on the TFT substrate can be used as a light shielding layer.
FIG. 1 shows in schematic cross section of a conventional active matrix liquid crystal display device with color filters mounted on a TFT substrate.
As shown in FIG. 1, TFT substrate 10 comprises transparent glass substrate 11 which supports on one major surface thereof a plurality of patterned data lines 12 extending parallel to each other, color layers 13 of color filters and transparent overcoat layer 14 which are successively deposited on the major surface of transparent glass substrate 11, and transparent pixel electrodes 15 disposed on the surface of overcoat layer 14 in association with the respective pixels. Data lines 12 are covered with color layers 13, and extend in a direction normal to the sheet of FIG. 1. Opposing substrate 20 comprises glass substrate 21 supporting on a transparent uniform common electrode 22 on one major surface thereof. TFT substrate 10 and opposing substrate 20 are spaced a given distance from each other with pixel electrodes 15 and common electrode 22 confronting each other. A liquid crystal layer 30 is sealed between TFT substrate 10 and opposing substrate 20. Each of data lines 12 is made of an opaque conductive material and serves to block gaps between two adjacent pixels against the entry of light. As well known to those skilled in the art, TFT substrate 10 also supports gate lines and TFTs associated with the respective pixels. The data lines are also referred to as video signal lines or drain lines and source lines, and the gate lines as scanning lines.
FIG. 2 shows an equivalent circuit of such an active matrix liquid crystal display device.
As shown in FIG. 2, pixel electrodes 15 and TFTs 41 which are associated with the respective pixels are arranged in a matrix form on TFT substrate 10. TFTs 41, which operate as switching elements, have gates connected to gate lines 42, drains connected to data lines 12, and sources connected to pixel electrodes 15. However, the sources of TFTs 41 may be connected to data lines 12, and the drains thereof to pixel electrodes 15. Common electrode 22 is grounded, and a liquid crystal layer sandwiched between common electrode 22 and one pixel electrode 15 serves as one pixel portion 40. On TFT substrate 10, gate lines 42 extend parallel to each other and perpendicularly to data lines 12. Equivalent pixel capacitors 43 are connected parallel to the respective pixel portions 40. Data lines 12 and gate lines 42 are driven respectively by drivers 44 and drivers 45.
It has been pointed out that the above conventional active matrix liquid crystal display device with the color filters on the TFT substrate has a smaller viewing angle than the active matrix liquid crystal display device with the color filters on the opposing substrate, even if it is provided with a phase difference compensation plate. Table 1 given below shows measured viewing angles in vertical and horizontal directions of active matrix liquid crystal display devices with color filters on TFT substrates and an active matrix liquid crystal display device with color filters on an opposing substrate. The values set forth in Table 1 were obtained with phase difference compensation plates used on these display devices.
TABLE 1Type9.4″ UXGA12.1″ SVGA12.1″ SVGAPixel pitch120 μm300 μm300 μmColor filterTFT substrateTFT substrateOpposingpositionsubstrateViewing angle 90 degrees 92 degrees 90 degrees(Vertical)Viewing angle 90 degrees105 degrees110 degrees(Horizontal)
The viewing angle referred to above is an angle in which the ratio of contrast between white and black display images is 10% or higher. As can be seen from Table 1, the vertical viewing angle remains substantially the same irrespective of whether the color filters are disposed on the opposing substrate or the TFT substrate. However, the horizontal viewing angle is much smaller with the color filters disposed on the TFT substrate than with the color filters disposed on the opposing substrate. This tendency manifests itself if the pixels are smaller.
The above phenomenon will be described in detail below with reference to FIG. 1.
It is assumed that the conventional active matrix liquid crystal display device shown in FIG. 1 is used in a normally white mode. If pixels disposed one on each side of data line 12 displays a black image, then when the liquid crystal display device is driven by a dot inversion driving process, since a voltage of +5 V is applied to one of the pixel electrodes and a voltage of −5 V is applied to the other pixel electrode, a strong lateral electric field is generated in a region above data line 12 of liquid crystal layer 30, causing directors (liquid crystal molecules) 31 to fall thereby to substantially display a white image in that region. Specifically, as indicated by A in FIG. 1, a white image is displayed in the region of the gap between pixel electrodes 15 and a region slightly extending from the gap into the pixel electrodes. These regions are combined as a region where light leaks. In the other region, directors 31 are erected parallel to the direction from pixel electrodes 15 to common electrode 22, and a black image is displayed. When the white image region is viewed from the front of the active matrix liquid crystal display device, it is visually recognized as a black region because light is blocked by data line 12. When the white image region is obliquely viewed, as indicated by the arrow B, light is not blocked by data line 12, and liquid crystal layer 30 is affected by light that passes only through light leakage region A. While the region should be visually recognized as the black region, since there is light passing through liquid crystal layer 30 as indicated by the arrow B, the contrast in the black region is lowered, resulting in a reduction in the intensity of black in the black region.
If the liquid crystal display device is a highly fine display panel with small pixel pitches, then because the ratio of light leakage regions to ordinary pixel regions tends to be larger than a display panel with greater pixel pitches, the contrast in the black region as obliquely viewed is reduced, resulting in a smaller viewing angle. The ordinary pixel regions are referred to as normal regions where liquid crystal molecules are vertically oriented to display a black image.
The above phenomenon can occur with respect to the gate lines. However, inasmuch as a relatively large voltage is applied to the gate lines at all times unlike the data lines, and pixel electrodes are of a rectangular shape that is elongate parallel to the data lines in a color active matrix liquid crystal display device, the above phenomenon is not so noticeable as with the data lines, and does not lead to a substantial reduction in the viewing angle and visual perception.
In order to prevent the contrast from being lowered and also to prevent the viewing angle from being reduced, Japanese laid-open patent publication No. 10-104664 (JP, 10104664, A), for example, discloses an arrangement in which data lines have an increased width and overlap pixel electrodes with an overcoat layer interposed therebetween. The disclosed arrangement, however, is disadvantageous in that because the data lines need to be extremely large in width in order to achieve a desired viewing angle, the aperture ratio is lowered, and the layout of TFTs and auxiliary capacitors is limited.